Lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product by two principal wood-pulping processes known as the sulfite process and the kraft process. In the sulfite pulping process, lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which subsequently result in chemical functions of the phenolic and carboxylic type. Kraft process lignin generally is isolated by acid precipitation from the black liquor of a kraft pulping process at a pH below the pKa of the phenolic groups.
Depending on conditions under which the lignin is precipitated, the precipitated lignin may be either in the form of free acid lignin or a lignin salt. If lignin is precipitated at a high pH, such as about 9.5 to 10, the lignin is obtained in the form of a salt. If this lignin is further processed by washing, acidifying to a low pH, such as about 2 to 5, and further washed so as to be substantially free of salt and ash-forming ingredients, free acid lignin, known as "A" lignin, is obtained. A monovalent salt of lignin, such as an alkali metal salt or an ammonium salt, is soluble in water, whereas free acid lignin and polyvalent metal salts of lignin are insoluble in water.
The high degree of chemical activity which is characteristic of lignin permits the preparation of many organic derivatives. Lignin by-products have been variously employed in other chemical compositions as surfactants, extenders, dispersants, reinforcers, absorbents, binders, sequestering agents, emulsifiers and emulsion stabilizers, and as a stabilizing and protective colloid. Lignosulfonate compounds, particularly sodium salts of lignosulfonates, have been employed as additives and dispersants in textile dyestuffs and printing pigments. Sodium salt sulfonated lignin by-products have been sold for many years under the trademark Indulin.RTM. by Westvaco Corporation of North Charleston, S.C.
Certain lignins have been utilized as a binding agent for water-based ink systems incorporating pigments and/or dispersed dyes. U.S. Pat. No. 2,525,433 discloses the use of lignin dissolved in a water-missible solvent as a binding material for a pigment in printing ink. The method of U.S. Pat. No. 2,525,433 requires the use of a solubilizing agent, generally an organic amine or alcohol, to completely dissolve the lignin particles. The resultant solution when used with pigments for ink application is limited to newsprint or other porous paper. There is no film-forming action of the lignin and no solid particles remain after the lignin is solubilized. Adhesion to coated paper, calendered paper and sized papers is poor with excessive rub-off resulting.
Lignin is recovered from the black liquor of a kraft pulping process by decreasing the liquor pH by acidification with carbon dioxide, organic or inorganic acids or bivalent metal salts. When the liquor pH is reduced from its normal pH of 13.0-13.5, the lignin precipitates as the sodium salt in the form of agglomerates of around 70 microns median diameter (700,000 angstroms (A)) or larger. The lignin-black liquor slurry is filtered to remove the liquor and the lignin particles are recovered as a filter cake. This cake is washed to remove the occluded black liquor and the purified lignin solids are acidified to pH 2.5 and further washed. The lignin in this form is known as "A" lignin and contains 2-3% inorganic salts through additional washing can lower the inorganics to 0.5% or less. This lignin can be reacted with various organic or inorganic agents to produce products useful in dyestuff dispersants, asphalt emulsifiers, concrete air entrainers, and other useful compounds.
The "A" lignin aggregates which are recovered from the kraft pulping process consist of a popcorn-like, porous structure in which the individual ultimate particles are fused at their points of surface contact to adjacent particles. The bonds between these individual particles are extremely strong, and attempts in the past to break up these aggregates by grinding into smaller particles have not been successful in obtaining submicron particles, i.e., particles having a median diameter of less than one micron (10,000 A). Wet grinding of lignin produces only a particle size as low as about 30 to 40 thousand angstroms diameter.
Early U.S. Pat. No. 3,223,697 discloses a method of producing lignin as a kraft wood-pulping by-product in small particle size by mechanically continuously propelling a stream of an aqueous solution of an alkaline lignin through a mixing zone of constricted cross-sectional area with sufficient velocity to create turbulent flow while continuously introducing a stream of a precipitant for the lignin to mix substantially instantaneously with the lignin solution to precipitate the lignin particles in the mixing zone in small particle sizes. However, such a method requires considerable processing equipment and careful control of temperatures and pressures at considerable economic expense, and the random shapes and sizes of the lignin particles produced by the method do not allow close packing or particle-particle juxtaposition necessary for proper film-forming and binding applications.
In the use of lignin by-products in the formulation of certain products and chemical compositions, it is desirable that the lignin particles be small enough that the particles may form continuous films, particularly in use of the lignin as a binder or coating in printing, in laminating paper, in forming rigid boards, as an adhesive for binding sawdust, woodchips, or plywood, and as a green strength enhancer for ceramic bodies or stain for wood surfaces.